Why we chose cork exterior insulation for our net-zero-energy house
Among the innovative—some might say weird—products we’re trying out at our Dummerston, Vermont farmhouse, none is more unusual than the expanded cork insulation we’re currently installing as a layer of exterior rigid insulation. As I mentioned in a blog last summer, cork insulation has a great story behind it.
Cork? You’ve got to be kidding!
I first learned about expanded cork insulation years ago when exploring the attic of a 1920s-era home in Brattleboro. I found a rigid boardstock insulation comprised of cork with plaster on one side. It was made by Armstrong, which was then a company making cork products but is today one of the world’s leading manufacturers of flooring and ceiling products.
It turns out that the product was invented by accident in 1893 in New York City by a boat builder, John T. Smith. The cork granules he used to fill life preservers became clogged in a large tin funnel, and that slipped into the coals of a fire used to steam oak staves. When the owner of the shop discovered the tin funnel the next morning he expected the cork to be burned up, but instead it had expanded to fill the form and solidified into a solid block.
Smith experimented with the process and patented it as Smith’s Consolidated Cork, which he licensed to Armstrong. It was used for several decades for insulating buildings—especially cold-storage buildings. The apple storage building at historic Scott Farm in Dummerston, built in the 1920s or ’30s, is insulated with this product.
Why I like cork
Cork is a remarkable material. It is the outer bark of a species of oak tree (Quercus suber) native to the Western Mediterranean region. This thick, spongy bark protects the trees from fire. It can be peeled off every nine or ten years, and grows back. The bark is still harvested in Portugal, Spain, Algeria, Morocco, Tunisia, France, Italy, and a few other countries much as it was 2,000 years ago.
The primary use for cork is for wine bottles. The “corks” we all know are punched out of the bark in a really simple process. The residual cork (about 65-70% of the material) is processed into granules that are processed into a wide variety of uses.
To make cork flooring, floor underlayment, and gaskets, the granules are glued together and sliced into thin layers. Cork makes a great flooring material, because it is soft underfoot (resilient) and it absorbs sound. You will often find it in libraries, for example, due to those acoustic properties. My aunt and uncle installed cork floors in their Connecticut house in 1951, and that flooring is holding up very well more than 60 years later.
Cork is produced from ecologically rich forests that support significant biodiversity, including the endangered European lynx.
Photo Credit: Alex Wilson
Avoiding foam insulation
The primary reason I’m excited about using cork insulation on our house is that I don’t like some of the chemicals used in conventional foam insulation. Extruded polystyrene is made with a blowing agent, HFC-134a, which is a very potent greenhouse gas that is contributing toclimate change, and nearly all foam insulation materials contain hazardous brominated or chlorinated flame retardants. I’ve most recently written about these concerns here.
Cork, by contrast, contains nothing but cork—nothing! As it is produced today by Amorim Isolamentos, S.A., the granules are poured into large vats and heated with steam in an autoclave at about 650°F for 20 minutes. The heat expands the granules by about 30% and releases a natural binder, suberin, that exists in the cork. There are no added ingredients.
Isn’t cork a limited resource, or isn’t there a cork blight?
I get these questions whenever I mention cork. As far as I can tell, these were rumors that were started by companies making synthetic bottle stoppers for the wine industry that were trying to take away market share from natural cork. No, to the best of my knowledge there isn’t a blight.
Cork is a somewhat limited resource, so cork insulation will never come to dominate the rigid insulation market. But the resource is not disappearing and clearly it is a renewable resource.
The sad part of the story is that as synthetic corks and screw-lid wine bottles have replaced traditional natural-cork bottle-stoppers, the demand for cork has dropped. I’m told that in some parts of the western Mediterranean region, cork oak forests are being cut down and the land converted to other uses.
Shipping cork to Vermont from Portugal
I’ll admit that shipping cork across the ocean is a significant downside. While ocean shipping is very energy-efficient (far more efficient than shipping over land), the fuel used—a low grade of diesel—is very dirty. I struggled with that as I thought about the use of this material for our house. I’ve reviewed an analysis Amorim Isolamentos has done on the carbon footprint of their material, and it’s not too bad.
Ultimately, I decided that by publicizing our use of this material I would help generate demand that might help preserve the cork forests. I don’t expect that the U.S. will ever become a huge market, but for people wanting natural and rapidly renewable building materials or who have chemical sensitivities, cork is an option that can be considered. (Relative to chemical sensitivities, care should be taken to make sure that there isn’t sensitivity to the odor of expanded cork, which has a somewhat smoky smell—no doubt due to the heating.)
Our use of cork insulation
We are installing cork as a layer exterior insulation on our farmhouse. The air barrier for the significantly rebuilt early-1800s house is a fully taped and airtight Zip sheathing layer. On the interior of that will be 7-1/2” of cavity-fill insulation. On the outside of the sheathing is the 6” layer of cork.
The building enclosure is designed so that the cavity-fill insulation layer can dry to the interior (if it ever gets wet), while the cork can dry to the exterior. On the outside of the cork will be a layer of housewrap (a high-performance German product, Pro Clima Solitex, distributed by 475 High Performance Building Supply), vertical strapping to create a rainscreen, and wood clapboard siding.
We ordered the cork with shiplap edges so that joints would not extend all the way through the material. We had debated ordering 3″ thick cork and overlapping the joints—and that strategy would have worked fine—but we decided that we could save on labor with the thicker panels.
Next week I’ll provide some specifics on how the cork insulation is being installed. Eli Gould and his crew have done a wonderful job at figuring out how to work with the stuff—some of the details are quite tricky.